1. Field of the Invention
The present invention relates to an ion current detection device provided in connection with an ignition device to detect the combustion state of an internal combustion engine based on an ion current inside a combustion chamber.
2. Description of the Related Art
In an internal combustion engine, control must be performed to prevent misfiring and abnormal combustion phenomena such as knocking and preignition (premature ignition). One method proposed to detect the combustion state of an internal combustion engine measures an ion current inside the combustion chamber and detects the combustion state based on the ion current.
More specifically, when a spark is produced at the spark plug and air/fuel mixture burns in the combustion chamber, the air/fuel mixture is ionized. When the mixture is in the ionized state, if a voltage is applied to the spark plug, an ion current flows. Abnormal occurrences such as knocking, preignition, and misfiring can be detected by detecting and analyzing this ion current.
Japanese Unexamined Patent Publication No. 8-200195, for example, discloses one such ion current detection device. In this device, a capacitor as an ion current generating source is charged to a given voltage by the secondary current that flows when the primary current in the ignition coil is shut off; then, a current that flows through a closed circuit consisting of the capacitor, the secondary winding of the ignition coil, the spark plug, and an ion current detecting resistor, after a spark discharge, is measured as a voltage across the ion current detection resistor.
In this ion current detection device, the ion current detection voltage increases as the resistance of the ion current detecting resistor increases. Here, a processing device, connected to the output side of the ion current detection device, performs prescribed processing using the ion current detection voltage as an input voltage. Since the processing device is mounted in a vehicle, a battery voltage is used as the supply voltage for the processing device. Therefore, if the resistance of the ion current detecting resistor is increased excessively, the input voltage, i.e., the ion current detection voltage, exceeds the supply voltage when an ion current larger than a certain value flows, and reaches saturation in the processing device. If this happens, not only does it become impossible to detect the high-frequency knock signal contained in the ion current, but discontinuities are caused in the ion current at saturation points, introducing large noise into the signal passed through a filter.
On the other hand, if the resistance of the ion current detecting resistor is reduced, noise associated with the ignition coil increases, degrading knock detectability. That is, after the end of the discharge at the spark plug, the ignition coil contains residual magnetic energy and attempts to discharge this energy, causing LC resonance through interaction with the stray capacitance on the high-voltage line. This LC resonance causes noise. Further, when the ion current flows into the ignition coil, this current flow triggers the generation of a very small LC resonance in the ignition coil, which also adds to the noise. The LC resonance frequency of the ignition coil is generally 4 to 8 kHz, which is very close to the knock frequency (6 to 8 kHz). As a result, once LC resonance occurs, it is difficult to separate its noise component from the knock signal component using a knock detection filter. Therefore, if the resistance of the ion current detection resistor is made too small, noise caused by the LC resonance cannot be attenuated, resulting in a degradation of the accuracy for the detection of knock and other abnormal combustion phenomena.